1. Field of the Invention
The present invention relates to a treatment method of spent uranium catalyst, and more specifically, to a method which can considerably reduce the volume of the spent uranium catalyst to be disposed of and simultaneously minimize secondary wastes that can be generated during the process of treating the spent uranium catalyst.
2. Description of the Related Art
Since the 1970s, U—SbOx/SiO2 catalyst which is supported by USbO5 or USb3O10, complex compounds of uranium (U) and antimony (Sb), on a porous silicon (support: SiO2), or UwSbxFeaAlbMocVdOz/SiO2 catalyst in which iron (Fe), aluminum (Al), molybdenum (Mo), vanadium (V), bismuth (Bi), as well as U and Sb are mixed have been developed in order to produce acrylonitrile (CH2═CHCN), a base material of synthetic fiber. These uranium catalysts use only depleted uranium having U-238 with U-235 mostly eliminated.
These uranium catalysts, which were used by Du Pont, or Union Carbide in U.S.A., or Sohio of Japan, etc. until approximately 2000, have been replaced by nonradioactive catalysts which are free of uranium since 2000. A South Korean private company had produced acrylic synthetic fiber using the uranium catalyst until 2004 and since then has used the nonradioactive catalyst, and consequently, approximately 7,100 drums (one drum is 200 liters) of spent uranium catalysts, which had been generated until the use of nonradioactive catalysts, are stored at the site. Although not accurate, the amount of the stored uranium waste at the acrylonitrile production sites in foreign countries, is also expected to be voluminous.
In regard to a domestic regulation on disposal of low and intermediate level radioactive wastes, Notification No. 205˜18 published by Korean Ministry of Science and Technology regulates alpha radioactivity of a solid radioactive waste to be 3,700 Bq/g or below, which corresponds to 14.6 wt % U for natural uranium and 25.2 wt % for depleted uranium. In order to discharge the radioactive wastes at the same level as the nonradioactive wastes into the environment, as exempt waste, the solid wastes need to have very low concentration of U, which is approximately 0.005 wt % or less.
Currently, the radioactivity of the spent uranium catalyst using the depleted uranium generated within South Korea conforms with the standards for transferring radioactive wastes to a radioactive waste disposal site to be operated in Gyeongju area in the future. Although the current disposal of the wastes costs approximately KRW 8.5 million per 200 L drum, the cost is expected to reach nearly KRW 10 million per the same-volume drum. Moreover, given that the volume increases due to additives added in the process of preparing the solid form for the disposal of the spent uranium catalyst, the final wastes to be disposed of add up to nearly 10,000 drums and thus the direct disposal cost can amount to KRW 100 billion.
The U used for the spent uranium catalyst is depleted uranium, which has little economic value, and therefore, if the U from the spent uranium catalyst is selectively and completely disposed of at a radioactive waste disposal site, the volume of the wastes to be disposed of can be reduced by up to 95% theoretically, and the disposal cost for the separated U can be approximately KRW 5 billion. As a means to use effectively the domestic radioactive waste disposal facilities to be constructed in the future and to minimize the secondary wastes for the reduction of the disposal cost for the spent uranium catalyst, an effective technology is needed for reducing the volume of the spent uranium catalyst by separating U exclusively from the spent uranium catalyst as much as possible while reducing the disposal cost.
Another concern is that, if acidic or alkaline solutions used to eliminate metals or Si, a supporter material, which are included in the spent uranium catalyst, are left after treatment of the spent uranium catalyst, much amount of secondary wastes to be disposed of can be generated due to the left acidic or alkaline solutions.
Therefore, a technology is necessary, to eliminate dissolved metal materials including uranium from the acidic and alkaline solutions after the process of eliminating uranium by using the acidic and alkaline solutions and then recover the acidic and alkaline solution by using electrodialysis from the mixture of several inorganic solutions used in the treatment process for the volume reduction of spent uranium catalyst.
Conventional technologies for reducing the volume of spent uranium catalyst for disposal and for preventing secondary wastes from being generated are introduced in the after-mentioned documents. Patent Document 1 introduces a method in which sodium carbonate or sodium hydrogen carbonate is mixed with the spent catalyst to dissolve Si, a main component of spent uranium catalyst; the mixture is reacted at a high temperature (1,000˜1,600° C.) to produce sodium silicate; water is added to the sodium silicate under 1˜10 atm and at a temperature of 10˜200° C. to create liquid sodium silicate; and the liquid sodium silicate and undissolved solid materials are separated by a solid-liquid separator. However, the method above focuses on how to separate the Si from the spent uranium catalyst, and therefore can overlook a point that part of U can be dissolved together in the alkali carbonate solution and then the U can remain in the secondary liquid waste generated from the process of treating the spent uranium catalyst. And since the method does not provide a measure of treating secondary liquid waste, reducing the volume of the spent uranium catalyst can be limited.
Patent Document 2 introduces a method in which, as a dry process, alkali carbonate is added to the spent uranium catalyst and stirred to react at a temperature of 1,000 to 1,600° C. in order to change the components of the spent catalyst into the form of alkali salt; or as a wet process, alkali hydroxide is dissolved in water and the spent uranium catalyst powder is mixed uniformly with the aqueous solution to react under the condition of 1 to 20 atm and at a temperature of 10 to 300° C. in order to change the components of the spent uranium catalyst into the form of alkali salt. However, this method overlooks the fact that uranium can be dissolved together with other components including Si in carbonate solution or alkali hydroxide solution and it can be intermixed with the dissolved Si solution. Consequently, the method above is practically limited in treating the spent uranium catalyst as it does not present an alternative to handling the Si precipitate contaminated with uranium and the uranium-bearing solution.
Patent Document 3 suggests a method in which spent uranium catalyst is heated at a temperature of 1,200 to 1,800° C. to separate and cool antimony oxide, molybdenum oxide, or volatile materials of the mixture of antimony oxide and molybdenum oxide, and to collect the powder, and remaining wastes from which the volatile material is separated and eliminated are heated at a reduced temperature of 600 to 1,300° C., and then a vitrifying agent is added to vitrify the remaining wastes in order to treat the spent uranium catalyst. But, the method above cannot be effective in reducing the volume due to a small content of the volatile materials in the spent uranium catalyst and in reducing the processing cost due to a larger amount of economic costs for the high-temperature treatment and the collection of the volatilized materials, compared with direct disposal cost of the spent uranium catalyst.
As stated above, technologies for treating the sparingly-soluble spent uranium catalyst have not been significantly developed. Referring to the Patent Documents 1, 2, and 3 mentioned above, dissolving the spent catalyst in the alkaline conditions at a high temperature/pressure causes weakness in the operating stability.
Accordingly, in the conventional technologies, the economic costs can increase and treating the uranium remaining in the secondary waste solutions can be challenging to be overcome.
In light of the situation above, there is an urgent need to develop environmentally-friendly technologies that can minimize the generation of secondary wastes and reduce the volume and weight of the spent uranium catalyst to be disposed effectively.